Nuclear logging techniques are commonly used in both wireline logging and logging while drilling operations. For example, neutron logging techniques are commonly employed to compute formation porosity. Gamma ray logging techniques are employed to compute bulk formation density, from which formation porosity can also be derived, possibly in combination with the neutron porosity measurement. Conventional bulk density measurements commonly make use of an isotopic source of gamma rays (also referred to as a chemical source), such as 137 Cs . Such radioactive chemical sources have obvious disadvantages from a radiation safety viewpoint and these disadvantages are of some concern in measurement or logging while drilling (MLWD) applications. Owing to these safety concerns (and potential security concerns as well), there is a desire in the oilfield services industry to replace the traditional 137 Cs source (see, for example, National Academy of Sciences, Radiation Source Use and Replacement: Abbreviated Version, The National Academies Press, 2008).
U.S. Pat. Nos. 5,608,215 and 5,804,822 to Evans et al disclose nuclear logging methods for measuring a formation density that employ an accelerator based neutron generator. Neutrons emerging from the accelerator interact with the tool, the borehole fluid, and the formation to produce gamma rays that can be detected elsewhere in the tool. These gamma rays may be thought of as being generated by a “secondary” gamma ray source (as opposed to a primary source such as the aforementioned chemical source). Neutrons may also be detected at the tool and used to correct for neutron attenuation effects on the secondary gamma ray source. The detected gamma rays (and neutrons) are analyzed to estimate a formation bulk density.
While the '215 and '822 patents disclose methods for estimating a formation bulk density without using a chemical source of gamma rays, the disclosed methods tend to be inaccurate. For example, these patents disclose that secondary gamma rays are produced via interactions of the neutrons with the logging tool, the borehole fluid, and the formation. It is further disclosed that these secondary gamma rays are used to compute the formation density. It will be readily apparent to those of ordinary skill in the art that gamma rays originating in the tool and the borehole fluid carry less information pertaining to the formation density than those from the formation. The failure to discriminate between gamma rays originating in the formation and gamma rays originating in the tool or the borehole fluid essentially averages all detected gamma rays, which can lead to significant errors in the estimated formation density.
Moreover, the secondary gamma rays can be generated via two distinct neutron interactions; inelastic scattering events and neutron capture events. The number of gamma rays produced via neutron capture events tends to be strongly influenced by the amount of hydrogen and the thermal neutron capture cross section of the formation. The number of gamma rays produced via inelastic scattering events is less dependent on these quantities and therefore tends to be more directly related to formation density. Odom et al in U.S. Pat. No. 5,900,627 attempt to eliminate gamma rays produced via neutron capture events by the use of a pulsed neutron generator. While such pulsing can eliminate many of the capture gamma rays, further improvements are needed to more fully discriminate between the inelastic and capture gamma rays.
Therefore there is a need in the art for an improved formation density logging technique that makes use of a neutron generator. In particular there is a need for a method that improves the accuracy of the measured formation density.